Heat sensitive materials for recording instruments



United States Patent Ofilice 2,855,266 Patented Get. 7, 1958 HEAT SENSITIVE MATERIALS FOR RECORDING INSTRUMENTS Raymond W. James, Lexington, Mass., assignor to Arthur D. Little, Inc., Cambridge, Mass., a corporation of Massachusetts No Drawing. Application February 16, 1953 Serial No. 337,219

7 Claims. (Cl. 346-1) This patent relates to the production of a material suitable for making continuous records and more particularly to a heat sensitive coated paper suitable for use by continuous recording devices.

Data supplied by measuring instruments of all types are normally recorded on strips or sheets of paper or Other suitable material which is usually sensitive to pressure or heat conveyed to the paper by a stylus or similar device. Since this type of paper for recording instruments is normally exposed to handling, to some external pressures, and to normal conditions of temperature and humidity, it is desirable to have a paper which is heatsensitive at temperatures above 250 F. and non-sensitive to lower temperatures, to pressure and to humidity.

It is an object of this invention to provide a recording material of the heat-sensitive type which is not sensitive to temperatures lower than 250 F. It is a further object to provide a heat-sensitive recording material whose time for response to heat may be varied according to the requirements imposed by the instruments with which it is to be used. Another object of this invention is to provide a heat-sensitive recording material sensitive to a wide-range of elevated temperatures. Still another object is to provide a heat'sensitive recording material which is completely stable ,to normal conditions of humidity and temperature and which thus remains dry and non-tacky under storage conditions. A still further object of this invention is to provide materials of this type which are inexpensive, easy to manufacture, durable, insensitive to pressurable contact and which have good keeping qualities. These and other objects will become apparent in the following discussion.

Recording paper of this type is particularly suitable for purpose of electrocardiographs because it avoids the usual photographic process for development; for spectrometers where results may be read directly; for continuous recording instruments in airplanes where changes in atmospheric Pressure makes the use of ink impractical; and, for a large number of applications where records are observed as they are being made.

Fundamentally, this invention consists of a coated strip of paper or other suitable support, comprising a mixture of a polymeric material having chlorine or bromine attached to a linear chain in the polymer and an inorganic metal compound, which mixture, when heat is applied, will turn black or a characteristic color. It is known that the addition of certain inorganic compounds to polymeric compounds containing chlorine or bromine attached to a linear chain will catalyze the heat degradation of these compounds. The application of heat by means of a stylus at an elevated temperature to mixtures of such polymeric compounds and inorganic compounds used as a film or coating on a supporting material gives a black or colored line which is permanently fixed in the coating or film. By varying the components of the mixture, the ratio of the components, and the pH of the coating mixture, it is possible to vary the times of response, the

temperature at which the color change takes place, and the colors produced.

The presence of inorganic compounds of magnesium, iron, cobalt, nickel, copper, Zinc, arsenic, molybdenum, silver, tin, antimony, and lead have been shown to act as catalysts in the thermal degradation of polymeric compounds containing chlorine or bromine attached to a linear chain. All of the above-mentioned, except the molybdenum and magnesium compounds, give a degradation product which is black. Molybdic acid gives a blue-green degradation product; while magnesium oxide gives a purple-red marking. It is possible, however, that if many more of the known compounds were used, other colors could be found.

The polymeric material used in the coating composition may be any polymer containing a linear chain to which is attached one or more chlorine or bromine atoms. A polymer containing fluorine only is not satisfactory, presumably because the C F bond is too strong.

The polymer may be used as a latex (suspended in water) or as a resin which is dissolved in a suitable solvent such as methyl ethyl ketone. If more flexibility is desired, a plasticizer may be employed to give the re quired degree of flexibility.

Compounds of magnesium, iron, cobalt, nickel, copper, zinc, arsenic, molybdenum, silver, tin, antimony, and lead have been found to be suitable inorganic additives. In order to insure that the material i insensitive to humidity changes, the inorganic compounds should be limited to the relatively water-insoluble compounds of the metals listed. Of the metals tried, the compounds of zinc gave the most rapid response time at lowest temperatures. The preferred zinc compounds are zinc carbonate and zinc oxide. Because there is some possibility that the Zinc carbonate will decompose under long-time storage and will liberate CO the zinc oxide is considered the best zinc compound for this purpose.

The time of response required to produce a mark at a given temperature may be lowered, and the temperature required to give a rapid or predetermined response may be decreased by maintaining a low pH during the degradation reaction. in using a polyvinyl chloride-zinc carbonate mixture, it was discovered that the pH change was from 6 to 2 in the degradation process. A mixture of 6 parts polymer and 4 parts zine carbonate was made up and to it was added 4 parts anhydrous NaOH and to another similar sample 4 parts (NHQ S O (a salt decomposing at 248 F. to liberate H 50 Minimum temperatures required to initiate the reaction were 335 F. for the one containing NaOH, 305 F. for the standard and 290 F. for the one containing (NHQ S O Response times at 450 F. were 6, 3 and 3 seconds, respectively. Thus, it appears that acidic conditions are desirable for bringing about the degradation reaction at lower temperature and in a shorter time. Other compounds (such as dimethyl aniline hydrochloride) which decompose to liberate free acid at temperatures below that at which degradation takes place may be used as additives. The preferred amount and type of acid-forming compound is one that in suflicient quantities will maintain the final pH of the degradation products below about3.

Varying the ratio of the inorganic compound to the polymeric material does not markedly affect either the temperature required to initiate the reaction or the speed of reaction at a given temperature. The ratio of latex (in terms of solid content) to inorganic compound may range from 3:7 to 7:3 by weight. However, a ratio of 4:6, or greater, results in the zinc powder flaking oil the recording material when handled. The preferred range for the zinc compounds is 4 parts by weight of zinc compound to 6 parts by weight of polymer (solid basis). At

this concentration a reaction begins to take place at 320 F., and at 370 approximately 6 seconds are required to produce a deep black.

The thickness of the heat-sensitive coating has little influence upon the performance of the recording material either as to the temperature at which the degradation action takes place or upon the response time. Thicknesses varying from 0.5 mil to over 2 mils have been successfully used. The thinner coating of about 0.5 mil is to be preferred in achieving a cheaper, lighter and more flexible finished product.

The material supporting the coating is preferably a relatively inexpensive flexible material such as paper or cloth, although metal or any other suitable base may be employed if desired. The choice of a supporting base will depend upon the weight, flexibility, durability, economy, and other characteristics sought in the final record ing material.

The coating itself may be white or slightly tinted, de-

pending upon the inorganic compound used, and may be applied without further coloring to give a white or lightly colored background. However, the coating may be colored by use of suitable pigments before application to give a stronger colored background for the black or colored lines formed.

, If the polymeric material is used in the form of a latex, the inorganic compound and acid-liberating additive, if used, are ball-milled, using the water of the latex as a milling medium. If the polymeric material is dissolved in a solvent, it is mixed in any suitable mixer with the inorganic compound and acid-liberating additive, if used. The dispersion of latex or solution is then applied to the supporting material in the form of a thin film by roller cooating, spraying, printing, floating, knife-coating or other method suitable to the consistency of the coating mixture and the nature of the supporting material.

Drying of the coating material may be accomplished by drying in open air, oven-drying at temperatures below 200 F., or by any other suitable method in which the temperature does not rise above 250 F.

The objects, aspects and features of this invention are further illustrated by, but not limited to, the following examples of practical embodiments of the invention.

Example I Twelve parts by weight of a polyvinyl chloride latex (50% polyvinyl chloride) and 4 parts by weight of zinc oxide were ball-milled for one hour, and the resulting suspension was knife-coated on a sheet of krat't white paper. This coated paper was oven-dried at 100 F. for 10 minutes. The resulting recording material, having a coating 0.5-1.0 mil thick, gave a black line instantaneously when marked with a stylus heated to 475 F. and began to turn black at temperatures as low as 320 F.

The usual temperature range for this heat-sensitive paper was therefore between 320 F. and 475 F., with re sponse times ranging between one minute and one second, respectively.

Example II Twelve parts by weight of a polyvinyl chloride latex (50% polyvinyl chloride, i. e., 6 parts by weight of polyvinyl chloride) and 4 parts by weight of zinc oxide and 4 parts by weight of ammonium persulfate were ballmilled for one hour and the resulting suspension was knife-coated on a sheet of lrraft white paper. This coated paper was ovcndried at l F. for 10 minutes to give a dry, ttotrtacky, heat-sensitive product. The resulting recording material having a coating 0.5-1.0 mil thick gave a black line instantaneously when marked with a stylus heated to 475 F. and began to turn black at temperatures as low as 290 F. The usual temperature range for this heat-sensitive paper was therefore between 290 F. and 475 F., with response times ranging between l minute and 1 second, respectively.

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4. Example Ill Six parts by weight of a polyvinyl chloride-polyvinyl acetate copolymer, 4 parts by weight of zinc oxide and 4- parts by weight of dimethyl aniline hydrochloride were mixed in a solution of methyl ethyl ketone and applied as a coating to paper as in Example I. Instantaneous marking took place at a stylus temperature of 475 F., and the material had a temperature range of 310 F. to 475 F. and a response time of 1 minute to 1 second, respectively.

Example IV Zinc carbonate was substituted for zinc oxide in Example I. The degradation reaction (black-line forma tion) was initiated at 290 F., and instantaneous marking took place at a stylus temperature of 475 F. The timeresponse range was from 1 minute to 1 second, respectively.

Example V Cupric acetate was substituted for zinc oxide in Example I. The coating itself was blue in color, and upon this background black-line formation was initiated at 325 F. The coating had a time-response of 4 seconds at 550 F.

Example VI Magnesium oxide was substituted for zinc oxide in Example I. A purple-red line formation was initiated at 340 F., and the coating had a time-response of 4 seconds at 550 F.

Example VII Ferrous phosphate was substituted for zinc oxide in Example I. The coating itself was gray in color, and upon this background black-line formation was initiated at 320 F. The coating had a time-response of 2 seconds at 550 F.

Example VIII Molybdic acid (H Mo0 was substituted for zinc oxide in Example I. A blue-green line was formed at 365 F, and the coating had a time-response of 5 seconds at 550 F.

Example IX Equal parts by weight of a polyvinyl chloride (suspended in water to form a latex) and zinc carbonate were ball-milled, and paper was coated with the mixture as described in Example I. Black-line formation was initiated at 320 F., and the coating had a time-response of 6 seconds at 370 F.

Example X Stannous chloride was substituted for zinc oxide in Example I. The coating itself was tan in color and upon this background black-line formation was initiated at 310 F. The coating had a time-response of 2 seconds at 550 F.

Example XI Natural rubber latex was brominated by the addition of hydrobromic acid. To this brominated polymer was added an amount of zinc oxide equivalent to a weight ratio of solids of 6 to 4, polymer to Zinc oxide. Paper coated with this material showed black-line formation at 390 F.

These above examples illustrate that wide ranges of time responses and temperatures, along with variations in the characteristic color ol. the degradation products forming the markings and in the color of the coating background, may be attained. it should be understood that the present disclosure is for the purpose of illustralion only, and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

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l. A heat-sensitive, dry, nontacky recording material,

comprising a supporting material and a coating, said coating being characterized by turning color upon the application of heat above a temperature of 250 F., said coating comprising polyvinyl chloride, zinc oxide and a compound which upon heating to a temperature between 200 F. and 300 F. liberates acid, said polyvinyl chloride and said zinc oxide being present in a weight ratio range of from 7/3 to 3/7 and said acidliberating compound being present in sufiicient quantity when heated to from 200 F. to 300 F. to maintain a pH of the final degradation product below about 3.

.2. A heat-sensitive, dry, non-tacky recording material, comprising a supporting material and a coating, said coating being characterized by turning color upon the application of heat above a temperature of 250 F., said coating comprising 6 parts by weight of polyvinyl chloride, 4 parts by weight of zinc oxide, and 4 parts by weight of ammonium persulfate, and being from 0.5 mil to 2.0 mils thick.

3. Method for recording the tracings of a stylus heated to a temperature above 250 F. comprising contacting said stylus with a heat sensitive, dry, non-tacky recording material, said recording material comprising a supporting material and a coating, said coating being characterized by turning color upon the application of heat above a temperature of 250 F., said coating consisting essentially of an organic polymeric material containing sub stituents selected from the group consisting of chlorine and bromine attached to a linear chain, and a compound of a metal selected from the group consisting of zinc, copper, iron, molybdenum, magnesium, cobalt, nickel, arsenic, silver, tin, antimony, and lead, said polymeric material and said compound being present in a weight ratio range of 7/3 to 3/ 7.

4. Method for recording the tracings of a stylus heated to a temperature above 250 F. comprising contacting said stylus with a heat sensitive, dry, non-tacky recording material, comprising a supporting material and a coating, said coating being characterized by turning color upon the application of heat above a temperature of 250 F., said coating consisting essentially of an organic polymeric material containing substituents selected from the group consisting of chlorine and bromine attached to a linear chain, a water-insoluble compound of a metal selected from the group consisting of zinc, copper, iron, molybde' num, magnesium, cobalt, nickel, arsenic, silver, tin, antimony, and lead, and a compound which upon heating to a temperature between 200 F. and 300 F. liberates acid, said polymeric material and said water-insoluble compound being present in a weight ratio range of 7/3 to 3/7 and said acid-liberating compound being present in sufficient quantity when heated to from 200 F. to 300 F. to maintain a pH of the final degradation product below about 3.

5. Method of making a heat-sensitive material for recording instruments, comprising the steps of forming an aqueous dispersion of an organic polymer material containing substituents selected from the group consisting of chlorine and bromine attached to a linear chain, and a compound of a metal selected from the group consisting of zinc, copper, iron, molybdenum, magnesium, cobalt, nickel, arsenic, silver, tin, antimony, and lead, applying said aqueous dispersion to a supporting backing and driving oif the water of said dispersion at a temperature below about F. to form a coating on a recording material suitable for making tracings when contacted by a recording instrument at a temperature above about 250 F.

6. A heat-sensitive recording material in accordance with claim 5, wherein the polymeric material is polyvinyl chloride and contains a plasticizer.

7. A heat-sensitive recording material in accordance with claim 5, wherein the inorganic compound is a zinc compound.

References Cited in the tile of this patent UNITED STATES PATENTS 1,844,199 Bicknell Feb. 9, 1932 1,939,232 Sheppard Dec. 12, 1933 2,141,126 Doolittle Dec. 20, 1938 2,436,216 Leatherman Feb. 17, 1948 2,591,368 McCarthy Apr. 1, 1952 2,600,134 Sowa et a1. June 10, 1952 2,610,920 Hopkinson Sept. 16, 1952 FOREIGN PATENTS 588,435 Great Britain May 22, 1947 705,417 Great Britain Mar. 10, 1954 

1. A HEAT-SENSITIVE DRY, NON-TACKY RECORDING MATERIAL, COMPRISING A SUPPORTING MATERIAL AND A COATING, SAID COATING BEING CHARACTERIZED BY TURNING COLOR UPON THE APPLICATION OF HEAT ABOVE A TEMPERATURE OF 250*F., SAID COATING COMPRISING POLYVINYL CHLORIDE, ZINC OXIDE AND A COMPOUND WHICH UPON HEATING TO A TEMPERATURE BETWEEN 200*F. AND 300*F. LIBERATES ACID, SAID POLYVINYL CHLORIDE AND SAID ZINC OXIDE BEING PRESENT IN A WEIGHT RATIO RANGE OF FROM 7/3 TO 3/7 AND SAID ACIDLIBERATING COMPOUND BEING PRESENT IN SUFFICIENT QUANTITY WHEN HEATED TO FROM 200*F. TO 300*F. TO MAINTAIN A PH OF THE FINAL DEGRADATION PRODUCT BELOW ABOUT
 3. 